gh-114914: Avoid keeping dead StreamWriter alive (#115661)

In some cases we might cause a StreamWriter to stay alive even when the
application has dropped all references to it. This prevents us from
doing automatical cleanup, and complaining that the StreamWriter wasn't
properly closed.

Fortunately, the extra reference was never actually used for anything so
we can just drop it.

3 files changed, 31 insertions, 10 deletions

diff --git a/Lib/asyncio/streams.py b/Lib/asyncio/streams.py
index df58b7a..3fe52db 100644
--- a/Lib/asyncio/streams.py
+++ b/Lib/asyncio/streams.py
@@ -201,7 +201,6 @@ class StreamReaderProtocol(FlowControlMixin, protocols.Protocol):
             # is established.
             self._strong_reader = stream_reader
         self._reject_connection = False
-        self._stream_writer = None
         self._task = None
         self._transport = None
         self._client_connected_cb = client_connected_cb
@@ -214,10 +213,8 @@ class StreamReaderProtocol(FlowControlMixin, protocols.Protocol):
             return None
         return self._stream_reader_wr()
 
-    def _replace_writer(self, writer):
+    def _replace_transport(self, transport):
         loop = self._loop
-        transport = writer.transport
-        self._stream_writer = writer
         self._transport = transport
         self._over_ssl = transport.get_extra_info('sslcontext') is not None
 
@@ -239,11 +236,8 @@ class StreamReaderProtocol(FlowControlMixin, protocols.Protocol):
             reader.set_transport(transport)
         self._over_ssl = transport.get_extra_info('sslcontext') is not None
         if self._client_connected_cb is not None:
-            self._stream_writer = StreamWriter(transport, self,
-                                               reader,
-                                               self._loop)
-            res = self._client_connected_cb(reader,
-                                            self._stream_writer)
+            writer = StreamWriter(transport, self, reader, self._loop)
+            res = self._client_connected_cb(reader, writer)
             if coroutines.iscoroutine(res):
                 def callback(task):
                     if task.cancelled():
@@ -405,7 +399,7 @@ class StreamWriter:
             ssl_handshake_timeout=ssl_handshake_timeout,
             ssl_shutdown_timeout=ssl_shutdown_timeout)
         self._transport = new_transport
-        protocol._replace_writer(self)
+        protocol._replace_transport(new_transport)
 
     def __del__(self, warnings=warnings):
         if not self._transport.is_closing():
diff --git a/Lib/test/test_asyncio/test_streams.py b/Lib/test/test_asyncio/test_streams.py
index 2109905..bf123eb 100644
--- a/Lib/test/test_asyncio/test_streams.py
+++ b/Lib/test/test_asyncio/test_streams.py
@@ -1130,6 +1130,31 @@ os.close(fd)
 
         self.assertEqual(messages, [])
 
+    def test_unclosed_server_resource_warnings(self):
+        async def inner(rd, wr):
+            fut.set_result(True)
+            with self.assertWarns(ResourceWarning) as cm:
+                del wr
+                gc.collect()
+                self.assertEqual(len(cm.warnings), 1)
+                self.assertTrue(str(cm.warnings[0].message).startswith("unclosed <StreamWriter"))
+
+        async def outer():
+            srv = await asyncio.start_server(inner, socket_helper.HOSTv4, 0)
+            async with srv:
+                addr = srv.sockets[0].getsockname()
+                with socket.create_connection(addr):
+                    # Give the loop some time to notice the connection
+                    await fut
+
+        messages = []
+        self.loop.set_exception_handler(lambda loop, ctx: messages.append(ctx))
+
+        fut = self.loop.create_future()
+        self.loop.run_until_complete(outer())
+
+        self.assertEqual(messages, [])
+
     def _basetest_unhandled_exceptions(self, handle_echo):
         port = socket_helper.find_unused_port()
 
diff --git a/Misc/NEWS.d/next/Library/2024-02-19-15-52-30.gh-issue-114914.M5-1d8.rst b/Misc/NEWS.d/next/Library/2024-02-19-15-52-30.gh-issue-114914.M5-1d8.rst
new file mode 100644
index 0000000..f7d392c
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2024-02-19-15-52-30.gh-issue-114914.M5-1d8.rst
@@ -0,0 +1,2 @@
+Fix an issue where an abandoned :class:`StreamWriter` would not be garbage
+collected.